Several unusual in vivo lymphoma lines were established and studied for surface and functional characteristics. The lines were derived from AKR/J strain mice and are noteworthy in that they exhibit characteristics of both T and B cells. When passaged into young normal AKR/J hosts, the splenic cells appear first like T-cell lymphomas and then acquire aspects of both T and B cells. Characteristics studied were Thy 1 antigen, Ia antigen, Fc receptor, complement receptor, mitogen responsiveness, and surface immunoglobulin. The Thy positive tumor cells in lymph node remain Ig negative. It is possible, 1) that tumor cells differentiate to Thy positive Ig positive cells in spleen, but not in lymph node or (2) that two tumor subpopulations exist, one of which grows in spleen and lymph nodes (Ig negative), while the other, (Ig positive), grows more slowly and selectively in spleen. Separation of Ig positive and Ig negative tumor cells remain Ig negative when passed into secondary hosts. The evidence is still inconclusive as to whether Ig positive tumor cells undergo a phase of Ig negativity in secondary recipients.